Ferritic heat-resistant steel

ABSTRACT

A HEAT-RESISTANT FERRITIC STEEL CONTAINS FROM 10% TO 15% CHROMIUM, THE LATTER&#39;&#39;S EFFECT IN MAKING THE STEEL RESISTANT TO OXIDATION BEING HELPED BY SMALL AMOUNTS OF ALUMINUM, SILICON, AND TO SOME EXTENT BY TITANIUM, THE LATTER, POSSIBLY ASSISTEDD BY OTHER STRONG CARBIDE-FORMERS, ALSO PREVENTING CHROMIUM LOSS AT THE STEEL&#39;&#39;S GRAIN BOUNDARIES, THE STEEL ADDITIONALLY CONTAINING SMALL BUT EFFECTIVE AMOUNTS OF CALCIUM, CERIUM OR OTHER RARE EARTH METALS USED SINGLY OR TOGETHER AND CAUSING ANY SCALE THAT FORMS TO BE ADHERENT AND FREE FROM FLAKING. THE STEEL IS PARTICULARLY ADAPTED FOR ELEVATED TEMPERATURE USE UNDER OXIDIZING CONDITIONS, PARTICULARLY WHEN THE SERVICE INVOLVES THERMAL SHOCK CONDITIONS, EXEMPLIFIED BY THE OPERATION OF INTERNAL COMBUSTION EXHAUST AFTER-BURNERS OR CATALYTIC REACTORS REQUIRED FOR POLLUTION CONTROL.

United States Patent Oflice 3,782,925 Patented Jan. 1, 1974 3,782,925 FERRITIC HEAT-RESISTANT STEEL Helmut Brandis and Rudolf Oppenheim, Krefeld, Germany, assignors to Deutsche Edclstahlwerke GmbH,

Krefeld, Germany No Drawing. Filed Dec. 13, 1972, Ser. No. 314,559

Claims priority, application Germany, Dec. 14, 1971, P 21 61 954.5 Int. Cl. C22c 39/14 US. Cl. 75- 124 12 Claims ABSTRACT OF THE DISCLOSURE A heat-resistant ferritic steel contains from to chromium, the latters eifect in making the steel resistant to oxidation being helped by small amounts of aluminum, silicon, and to some extent by titanium, the latter, possibly assisted by other strong carbide-formers, also preventing chromium loss at the steels grain boundaries, the steel additionally containing small but effective amounts of calcium, cerium or other rare earth metals used singly or together and causing any scale that forms to be adherent and free from flaking. The steel is particularly adaped for elevated temperature use under oxidizing conditions, particularly when the service involves thermal shock conditions, exemplified by the operation of internal combustion exhaust after-burners or catalytic reactors required for pollution control.

BACKGROUND OF THE INVENTION Many products, when in service, must operate at elevated temperatures under oxidizing conditions, examples being structural elements and inserts of furnaces, combustion chambers, metal annealing furnaces, and the like.

A particularly important example is provided by pollution-controlling automotive-vehicle exhaust after-burners or catalytic reactors which must be made in large numhers at the least possible cost and which, when in service, are subjected to thermal shock and corrosion in addition to temperatures of up to around 1000 C. under oxidizing conditions.

To be satisfactory when subjected to such conditions, a steel should be, in general, resistant to scaling, have thermal co-efiicients of expansion as low as possible so the products do not change in shape destructively at temperatures varying from room temperatures to the higher temperatures involved by their intended use, be resistant to corrosion, and have the ability to cause any scale that does form on its surface to be a tightly adherent layer having little or no tendency to flake off and cause trouble. In most instances the steel should be capable of production at the lowest cost possible, making the use of large amounts of the more expensive alloying metals undesirable.

In addition, certain products, particularly those referred to hereinabove for automotive exhaust pollution control, must operate under repeated widely varying and rapidly changing temperatures. An automotive after-burner or catalytic converter is required to go from the vehicles ambient temperature, and it may be wintertime, to temperatures ranging around the 1000 C. area and back to the initial temperature, a number of times a day. If the steel used results in the formation of a loose scale that flakes or cracks off and falls under the thermal shock conditions involved, such a device becomes impractical because the scale interferes with its operation. If the steel used thermally expands and contracts excessively, the device is not structurally stable and cannot operate properly.

DESCRIPTION OF THE PRIOR ART To meet such service conditions, both ferritic and austenitic stainless steels have been used, particularly when the service conditions involve heating up to the 1000 C. area.

The ferritic steels normally contain a minimum of 18% chromium, possibly together with aluminum or silicon. In addition to 18% or more of the chromium, the austenitic stainless steels contain relatively large amounts of nickel such as up to 36% in some instances. These austenitic stainless steels are, of course, scale-resistant but their large amounts of high-cost alloying metals make them very expensive.

Even so, the highly alloyed chromium-nickel austenitic stainless steels have not performed satisfactorily when used for the construction of automotive after-burners or catalytic reactors. The just-mentioned terms are here used to mean any of the devices intended for use to free internal combustion engine exhaust gases from components considered to be obnoxious or harmful to health and the operation of which requires high temperatures.

Such devices are ordinarily made with the metal in sheet form with seams secured by welding. The prior art highly alloyed steels have high thermal expansion properties so that under the greatly varying operating temperatures the products shape stability is unsatisfactory. Plainly, these steels large cost is an obstacle considering the competitive nature of the automotive field and the fact that automotive vehicles are produced in such very great numbers.

There have been efforts to produce satisfactory, lessexpensive alloys and certain chromium-nickel steels have proven adequate in many respects for use under the conditions described; even so, such alloys have had a thermal expansion co-eflicient too high for structural stability of the devices into which they are fabricated, particularly in the case of automotive afterburners.

SUMMARY OF THE INVENTION The object of the present invention is to provide a heat resistant steel which may be used generally when condi tions such as described hereinabove are involved, but which is particularly adapted for use in the construction of internal combustion engine exhaust after-burners and the like, and which does not involve the use of large amounts of the more expensive alloying elements, while having a satisfactory combination of scale resistance while causing scale that does form to be tightly adherent, good structural stability when subjected to widely varying temperatures, and little or no tendency to be brittle when heated to the temperatures involved by the use of Welding during the manufacture of after-burners and encountered when they are in service.

The above object is attained by a ferritic stainless steel consisting essentially of only from 10% to 15% chromium and small amounts of aluminum, silicon, titanium and one of the rare earth metals, particularly calcium and/or cerium, all being used within limited percentage ranges specified hereinafter, the balance being iron and the usual impurities. The steel may be modified by additional alloying elements, used in relatively small amounts.

In this new steel the chromium provides the resistance to oxidation, assisted by the aluminum and silicon and to some extent by the titanium, the latter in addition, optionally aided by other strong carbide formers, bonding with the carbon contained by the steel and preventing chromium impoverishment at the steels grain boundaries; and recognizing that under the service conditions described, the formation of some scale is to be expected, the calcium, cerium and/or other rare-earth metal, causing such scale to very firmly adhere to the steel so that it does not crack or flake off during abrupt temperature changes to be expected, particularly in the case of the operation of after-burners.

3 DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout the present specification and in the claims all references to percentage are by weight. With this understanding the new steel of the present invention basically has the following composition:

Percent Carbon 0.02 to 0.15 Chromium 10.0 to 15.0 Aluminum 1.0 to 3.5 Silicon 0.8 to 3.0 Titanium 0.3 to 1.5 Total calcium, cerium and/ or other rare earth metals 0.01 to 0.5 Total of niobium, tantalum and/ or zirconium to 1.0 Nitrogen 0 to 0.10 Manganese 0 to 1.0 Nickel 0 to 1.0 Iron (with the usual impurities including sulfur) Balance and in which the sum (C+N) amounts to at least 0.05% and at most to 0.20%, the sum (Al+Si) amounts to at least 2% and at most to 5%, and the sum with the proviso of a free titanium content of at least 0.2% not bonded by the carbon, nitrogen and/ or sulfur.

Within the above ranges preferred embodiments of the invention are described by the following:

Thus, preferably the carbon range is restricted to 0.04 to 0.08% and the chromium range to 11 to 13%. Less than chromium does not provide suflicient resistance to oxidation while more than 15% is not needed in the steel of the present invention. The influence of the chromium is assisted by the aluminum, the latter in this preferred embodiment having a range of from 1.8 to 2.5%. The silicon also assists the chromium in performing its intended function, and in this preferred form the silicon is restricted to a range of 0.8 to 1.5%; and the aluminum and silicon contents are adjusted relative to each other so that the aluminum content is about double the silicon content.

The titanium, which also assists the chromium, in this preferred form, may be within the full 0.3 to 1.5% range, because the titanium being a strong carbide former, also functions to tie up or combine with the carbon in the steel to prevent the formation of chromium carbides which would result in chromium impoverishment at the grain boundaries of the steel such as would make the steel subject to intergranular corrosion. An afterburner, when in operation, is subjected to corrosion because of the condensate which for-ms on its interior. Other strong carbide formers such as niobium (columbium), tantalum and zirconium, used either singly or together to provide a content within a 0.2 to 1.0% range, may be added to the titanium to tie up the carbon and in addition, because of the low solubility of the carbides they form, to also prevent grain coarsening during the welding required for the fabrication of after-burners, a formation of a coarse grain structure near the welding area resulting in undesirable brittleness.

For the titanium to assist the chromium in connection with oxidation resistance, it must be used to provide at least 0.2% excess beyond the amount needed, taking into account the action of the possible use of the other strong carbide formers as well as nitrogen and sulfur, to combine with the carbon in the steel. Therefore, in the preferred form, the steel should contain at least this 0.2% uncombined titanium.

In case the steel is used in the form of relatively thick or massive construction, it is preferred to add the nitrogen from an effective amount up to an amount of 0.10%, because the special nitrides it forms keep the steels grain size relatively fine as compared to that to be expected in the absence of the nitrogen.

To assure the formation of a firm adhered scale on the steel which will not crack or flake off during abrupt temperature changes or thermal shock conditions, the use of the calcium and/or cerium and/or other rare earth metals within the range of 0.1 to 0.5% is not only preferred but is of decisive importance. They may be used either singly or together. In this preferred embodiment if calcium is used, the range is from 0.05% to 0.2%; and if cerium or other rare earth metals are used either singly or together, their preferred range is from 0.03 to 0.1%. Within the range of 0.1 to 0.5% these rare earth metals may be used in any combination, singly or together.

Now it can be seen that this new steel does not require the use of large amounts of the more expensive alloying metals. It may be produced as a wrought steel having good machining properties. It is a ferritic stainless steel but even with prolonged heating it is free from embrittlement due to the intermetallic compound, sigma phase, or in the 425 C. range where prior art ferritic chromium steels having a chromium content corresponding to that of this new steel, become brittle with prolonged heating. Its co-eflicient of thermal expansion is adequately low to permit it to be made into structural elements such as after-burners, of good stability regardless of rapid heating and cooling stressing. Its good resistance to oxidation is retained at temperatures up to about 1100" C. and scale that forms is tightly adherent to the steel and does not flake off with changing temperatures.

Although this new steel can be used for many purposes, it is particularly adapted for use for making after-burners in view of the conditions they encounter when in service and the need for producing them in very large quantities at the minimum possible cost.

What is claimed is:

1. Ferritic heat-resistant steel by weight consisting essentially of:

Percent Carbon 0.02 to 0.15 Chromium s 10.0 to 15.0 Aluminum 1.0 to 3.5 Silicon 0.8 to 3.0 Titanium 0.3 to 1.5 Total calcium, cerium and/or other rare earth metals 0.01 to 0.5 Total of niobium, tantalum and/ or zirconium 0 to 1.0 Nitrogen 0 to 0.10 Manganese 0 to 1.0 Nickel 0 to 1.0 Iron (with the usual impurities including sulfur) Balance and in which the sum (0+N) amounts to at least 0.05 and at most to 0.20%, the sum (Al+Si) amounts to at least 2% and at most to 5%, and the sum with the proviso of a free titanium content of at least 0.2% not bonded by the carbon, nitrogen and/or sulfur.

2. The steel of claim 1 excepting the carbon content is 0.04 to 0.08%, the chromium content is 11 to 13%, the aluminum content is 1.8 to 2.5% and the silicon content is 0.8 to 1.5%.

3. The steel of claim 1 in which the aluminum content is twice that of the silicon content.

4. The steel of claim 2 in which the aluminum content is twice the silicon content.

5. The steel of claim 1 excepting that the calcium content is 0.05 to 0.2%.

6. The steel of claim 1 excepting that the cerium and/ or other rare earth metal content is 0.03 to 0.1%.

7. The steel of claim 5 in which the aluminum content is twice the silicon content.

8. The steel of claim 6 in which the aluminum content is twice the silicon content.

9. The steel of claim 1 excepting that the calcium content is 0.05 to 0.2% and/or the cerium and/or other rare earth content is 0.03 to 0.1%

10. The steel of claim 2 excepting that the calcium References Cited UNITED STATES PATENTS 3,272,623 9/1966 Crafts 75-l24 3,674,469 7/1972 Stewart 75-424 3,690,869 9/1972 Potak 75124 3,719,495 3/1973 Cointe 75-124 HYLAND BIZOT, Primary Examiner UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION l n fls) Helmut Brandis and Rudolf Oggenheim It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Abstract, Line 22, change "adaped" to --adapted-- in column 3, line 26, change "%Ti;O.5(%Nb %Zr2+0.25 52%;" I 0T8.

In column 3, line 30, change "amounts to at least I 4(%C)+3.5(%N)+3(%S)+O. 6%" to --amounts to at least I(%C)+3.5(%N)+3(%S)+0.2%, and

at most to 4(%C)+3.5(%N)+3 (%S)+o.6%,

I column line 59 (Claim 1), change "(O+N)" to ---(C+N)---- In column I, line 63 (Claim 1), change "(%Ti)O.5(%N"b+%Zr)+ O.25(%Ta) to I --(%Ti)+o.5(%Nb+%Zr)+O.25(%Ta)- column 6, line 2 (claim 10) change "earth metal content I is 0.1%" to --earth metal content is 0.03 to O.l%--

Signed and sealed this 23rd day of July 1974 (SEAL) Attest:

MCCOY M. GIBSON, JR. I 0. MARSHALL DANN Attesting Officer Commissioner of Patents ='OFIM PO-1050 (10-69) USCOMM'DC 376 P6D fl' U45. GOVERNMENT PRINTING OFFICE: Ill! 0 36-JJ4, 

